1. Field of the Invention
The present invention relates to a torque distribution control apparatus for a vehicle, for controlling the torque distribution ratio between two left and right driving wheels, or the torque distribution ratio between four front, rear, left and right driving wheels.
2. Description of the Prior Art
A differential mounted in a power transmitting system of a vehicle is constructed to absorb a difference in rotational speed produced between the left and right wheels during turning of the vehicle and distribute the torque of an engine at an appropriate ratio to the left and right wheels. However, a common differential is accompanied by a problem that it is operated by a difference between loads applied to the left and right wheels and hence, when one of the wheels has run onto a road surface having a small friction coefficient, so that it has been raced, the amount of torque transmitted to the other wheel is decreased, or the transmission of the torque is blocked.
In order to avoid such disadvantage, a torque distribution control apparatus has been proposed in Japanese Patent Application Laid-open No. 94421/87, in which a hydraulic clutch is disposed in each of power transmitting systems connected to the left and right driving wheels, so that a torque suitable for a now operational condition is distributed to the left and right driving wheels to improve the turning performance by independently adjusting the engagement forces of the pair of hydraulic clutches on the basis of a steering angle and a vehicle speed.
A torque distribution control apparatus is also known from Japanese Patent Application Laid-open No. 525/91, which controls the torque distribution to distribute the torque of an engine to front driving wheels and rear driving wheels at a predetermined longitudinal torque distribution ratio, and controls the torque distribution to distribute at least one of the torque distributed to the front driving wheels and the torque distributed to the rear driving wheels to the left driving wheels and the right driving wheels at a predetermined lateral torque distribution ratio. This torque distribution control apparatus maintains constant the longitudinal torque distribution ratio between the front driving wheels and the rear driving wheels, or performs a feed-forward control thereof in accordance with the torque of the engine, and performs a feed-back control of the lateral torque distribution ratio between left rear driving wheel and the right rear driving wheel in accordance with a vehicle speed and/or a lateral acceleration, thereby enhancing the turning performance of the vehicle.
In any of the above prior art control apparatus, however, the torque distribution ratio is only feed-forward controlled on the basis of the traveling condition of the vehicle and therefore, a control of a high accuracy is difficult, and it is necessarily not possible to match the actual turning condition of the vehicle with a turning condition desired by a driver.